1. Field of the Invention
The present invention relates to a ferroelectric capacitor, a semiconductor device equipped with the ferroelectric capacitor and fabricating methods thereof.
2. Description of the Related Art
A ferroelectric memory that uses a ferroelectric capacitor, in view of capability of rapid data recording and random accessing, is expected as a novel nonvolatile memory.
A ferroelectric memory is a memory that has a cell structure that includes a ferroelectric capacitor including a ferroelectric film and a transistor and utilizes reversal of an electric field due to spontaneous polarization of the ferroelectric film that stores and memorizes electric charges and retention function thereof (Patent Document Nos. 1 and 2.).
As the cell structure of the ferroelectric memory, there are mainly two types, namely, a planar type and a stack type.
In the planar type, a lower electrode film of a ferroelectric capacitor simultaneously plays the role of a plate line, and an upper electrode film is electrically connected to a diffusion layer of a transistor disposed side by side with the ferroelectric capacitor.
On the other hand, the stack type has a cell structure in which, above the transistor, a ferroelectric capacitor is stacked. That is, an upper electrode film of the ferroelectric capacitor combines with a plate line or is electrically connected to a plate line, and a lower electrode is electrically connected through a metal plug to a diffusion layer of a transistor disposed below the lower electrode. Accordingly, the stack type can reduce a cell area than the planar type and can be applied to a more miniature design rule.
[Patent Document No. 1]
JP-A No. 5-63203
[Patent Document No. 2]
JP-A No. 2002-289802
However, there are difficulties putting a stack type ferroelectric memory having a fine design rule into practical use, because of occurrence of various problems described below.
The stack type ferroelectric memory, as mentioned above, has a structure in which a ferroelectric capacitor is formed to a metal plug.
However, ordinarily, in the case of a ferroelectric film being deposited, in order to crystallize the ferroelectric film, high temperature heating in an oxygen atmosphere has to be performed.
Accordingly, when a film for forming a ferroelectric film is deposited, to an insulating layer formed below a lower electrode film, oxygen is diffused. As a result, a plug buried in the insulating layer is oxidized, and, in some cases, the continuity between the lower electrode film and a diffusion region cannot be attained (problem (1)).
In this connection, a method is proposed in which in depositing a ferroelectric film, a crystallizing temperature is set at a temperature the same or less than an oxidizing temperature of the plug material. However, in this case, the crystallization of the ferroelectric film becomes insufficient, resulting in a decrease in the reliability of the ferroelectric capacitor.
In this connection, as a method of inhibiting the plug from being oxidized, a method is proposed in which in processing a stack type ferroelectric capacitor, after a lower electrode film, a ferroelectric film and an upper electrode film are sequentially deposited on a plug, these films are collectively etched.
In this case, high temperature heating can be applied in a state where a lower electrode film that is oxidation resistant is not processed; accordingly, the oxidation resistance of a plug becomes very high.
However, on an end face of the ferroelectric film processed by the etching, in some cases, a damaged area is formed. The damaged region is a degenerated region that includes crystal structure irregularity and compositional change formed during the etching by a reaction between a ferroelectric material on an etching surface and a reaction gas, or an intermediate reaction product formed by a reaction between the upper electrode and lower electrode materials and a reaction gas.
As a result, in some cases, owing to the occurrence of such damaged region, a normal operation of the ferroelectric capacitor is disturbed, and the reliability of the ferroelectric capacitor cannot be secured (problem (2)).
Accordingly, the object of the invention is to solve the above problems (1) and (2) and thereby to provide a ferroelectric capacitor that can secure high reliability, a semiconductor device equipped with the ferroelectric capacitor and methods of fabricating these.